1. Field of the Invention
The present invention relates to a vibration actuator utilizing a synthesized vibration of an elongation-contraction vibration and a torsional vibration. More specifically, the present invention relates to a vibration actuator of so-called different mode degenerate type having a vibration element adhered to electromechanical converting elements and adapted to generate plural vibration modes, and a relative movement member maintained in pressure contact with the vibration element. In particular, the present invention relates to a vibration actuator capable of improving mechanical strength and driving characteristics without sacrificing the driving efficiency and allowing exact detection of the status of the generated vibrations.
2. Related Background Art
FIG. 1 is a perspective view of a conventional longitudinal-torsional vibration actuator.
In a conventional vibration actuator, a stator 1 is composed of two cylindrical vibration elements 2, 3 between which a piezoelectric element 4 for torsional vibration is sandwiched, and a piezoelectric element 5 for longitudinal vibration is provided on the vibration element 3. The piezoelectric element 4 for the torsional vibration is polarized in the circumferential direction, while the piezoelectric element 5 for the longitudinal vibration is polarized in the vertical direction. A relative movement member (rotor) 6 is provided on the piezoelectric element 5 for the longitudinal vibration.
The vibration elements 2, 3 and the piezoelectric elements 4, 5, constituting the stator 1, are affixed by rotatably fastening onto a screw portion formed on a shaft 7. The rotor 6 is rotatably supported by the shaft 7, by means of a ball bearing 8 provided at the center. A nut 9 is screwed on an end of the shaft 7, and a spring 10 is provided between the ball bearing 8 and the nut 9. As indicated by an arrow F, the rotor 6 is maintained in contact with an end face of the stator 1 under pressure, by the function of the spring 10.
The piezoelectric element 4 for the torsional vibration and the piezoelectric element 5 for the longitudinal vibration are driven by a driving voltage of a same frequency generated by a generator 11, schematically shown in FIG. 1, with a phase control by a phase shifter 12.
The piezoelectric element 4 for the torsional vibration generates a mechanical displacement for causing the rotation of the rotor 6. On the other hand, the piezoelectric element 5 for the longitudinal vibration performs a clutch function, for periodically varying the frictional force between the stator 1 and the rotor 6 in synchronization with the torsional vibration generated by the piezoelectric element 4, thereby converting the vibration into a one-directional movement.
FIG. 2 is an exploded perspective view of the stator 1 of the vibration actuator shown in FIG. 1.
As the piezoelectric element 4 for the torsional vibration has to be polarized in the circumferential direction, the piezoelectric material is divided into 6 to 8 sector-shaped pieces as shown in FIG. 2, which are individually polarized in the circumferential direction and then assembled again into annular shape. Electrode 13 is provided thereon.
However, in the conventional vibration actuator shown in FIGS. 1 and 2, it has been difficult to obtain sufficient precision during assembly of the piezoelectric element 4 for the torsional vibration into the annular shape. For this reason, the mutual contact between the piezoelectric element 4 and the vibration element 3 or between the piezoelectric elements 4 is hindered. As a result, the vibration of the piezoelectric elements 4 cannot be sufficiently transmitted to the vibration elements 2, 3, whereby the performance of the vibration actuator is deteriorated.
Also the piezoelectric element 4 for the torsional vibration, being assembled by adhering sector-shaped pieces, requires a thickness of at least several millimeters in order to obtain a sufficient adhesion strength. For this reason the distance between the electrodes of the piezoelectric element 4 becomes large, thus requiring a high voltage in order to provide the piezoelectric element 4 with an electric field necessary for driving.
On the other hand, the area of the piezoelectric element 4 for the torsional vibration and that of the piezoelectric element 5 for the longitudinal vibration is approximately equal to or smaller than the cross section of the vibration element 2 or 3. Also, for passing the shaft 7, the piezoelectric element 4 for the torsional vibration and the piezoelectric element 5 for the longitudinal vibration have to be provided with a hole at the center. Consequently the area of each of the piezoelectric element 4 for the torsional vibration and that of the piezoelectric element 5 for the axial vibration becomes even smaller, so that it has been difficult to obtain a higher torque and a higher revolution in the actuator.
For this reason, the present applicant has proposed, for example in the Japanese Patent Application Nos. 6-180279 and 6-275022, a vibration actuator employing a vibration element of different-mode degenerate type, utilizing a first-order elongation contraction (longitudinal) vibration and a first-order (or second-order) torsional vibration.
FIGS. 3A, 3B and 3C are views showing the driving principle of such vibration actuator, generating an elliptical movement on a driving face of the vibration element by the combination of a longitudinal vibration and a torsional vibration, wherein FIGS. 3A and 3B are respectively a plan view and a lateral view, and FIG. 3C is a wave form chart showing the longitudinal and torsional vibrations generated in the vibration element.
A vibration element 21 is composed by combining two semicircular cylindrical elastic halves 22, 23 of a form obtained by vertically dividing a hollow cylindrical elastic member by a plane containing the rotary axis thereof. Between the two semicircular cylindrical elastic halves 22, 23 there are adhered four piezoelectric elements 24, 25, two on each side, constituting the electromechanical converting elements, and, upon excitation of the piezoelectric elements 24, 25 by drive signals, there are generated a first-order torsional vibration and a first-order longitudinal vibration in the vibration element 21 as shown in FIG. 3C.
If the longitudinal and torsional vibrations generated in the vibration element 21 have approximately same resonance frequencies, such longitudinal and torsional vibrations are simultaneously generated in the vibration element (such state being hereinafter called "degenerate" state), and a driving face D, which is an end face of the vibration element 21, causes an elliptical movement in which a point orbit is an ellipse.
The elliptical movement thus generated drives a moving element 26, constituting the relative movement member maintained in pressure contact with the driving face D, by rotary motion of the moving element 26.
FIGS. 4A and 4B are respectively a plan view and a lateral view, showing the details of the structure of the vibration element 21.
The vibration element 21 is composed by combining two semicircular cylindrical elastic halves 22, 23 of a form obtained by vertically dividing a hollow cylindrical member 20, and between divided faces 22a, 23a of the elastic halves 22, 23 there are inserted piezoelectric elements 24, 25 and electrode plates 27, 28, 29 in a laminated state as shown in the drawing.
The piezoelectric elements 24, 25 are provided in two groups, each of which has four layers. Among such four layers, two layers (piezoelectric element 25 for torsional vibration) are made of a piezoelectric material utilizing a piezoelectric constant d.sub.15, while other two layers (piezoelectric element 24 for longitudinal vibration) are made of a piezoelectric material utilizing a piezoelectric constant d.sub.31. The piezoelectric element 25 utilizing a piezoelectric constant d.sub.15, for the torsional vibration, generates a shear deformation in the longitudinal direction of the vibration element 21.
The piezoelectric element 25 for the torsional vibration generates a torsional displacement in the vibration element 21 under a voltage application, while the piezoelectric element 24 for the longitudinal vibration generates a longitudinal shear deformation in the vibration element 21 under a voltage application.
Thus, in response to a sinusoidal voltage input to the piezoelectric element 25 for the torsional vibration, the vibration element 21 generates a torsional vibration, and, in response to a sinusoidal voltage input to the piezoelectric element 24 for the longitudinal vibration, the vibration element 21 generates an elongation-contraction (longitudinal) vibration.
FIGS. 5A and 5B show another conventional example of the different-mode degenerate vibration element.
More specifically, FIGS. 5A and 5B are respectively a plan view and a lateral view of a vibration element 31 of different-mode degenerate type, to be employed in a vibration actuator.
The vibration element 31 is provided with piezoelectric elements 34, 35 constituting the electromechanical converting elements for converting electric energy into a mechanical displacement, and a hollow cylindrical elastic member 30 generating an elongation-contraction vibration and a torsional vibration by the oscillation of these piezoelectric elements 34, 35.
The hollow cylindrical elastic member 30 is formed by combining two elastic halves 32, 33 of a form obtained by vertically dividing a hollow cylindrical member with a plane containing the central axis. On the external periphery of the hollow cylindrical elastic member 30 there is formed a smaller diameter portion 30a of a groove shape, which defines a first larger diameter portion 30A and a second larger diameter portion 30B. Therefore, on the external periphery of the cylindrical elastic member 30 there are in succession formed, along the central axis thereof (vertical direction in FIG. 5B), the first larger diameter portion 30A, the smaller diameter portion 30a and the second larger diameter portion 30B.
Between divided faces 32a, 33a of the elastic halves 32, 33, there are sandwiched four layers of piezoelectric elements 34, 35, each having two layers, and electrode plates 37, 38, 39 for applying drive voltages to the piezoelectric elements 34, 35.
The piezoelectric elements 34, 35 mounted between the divided faces 32a, 33a are composed of two layers each or four layers in total, in which the piezoelectric elements 35 of two layers generate a shear displacement in the longitudinal direction of the elastic halves 32, 33, utilizing the piezoelectric constant d.sub.15, while the piezoelectric elements 34 of remaining two layers generate an elongation-contraction displacement in the longitudinal direction of the elastic halves 32, 33, utilizing the piezoelectric constant d.sub.31.
The elastic halves 32, 33 generate a torsional displacement in response to the application of a drive voltage to the piezoelectric elements 35, while they generates a longitudinal displacement in response to the application of a drive voltage to the piezoelectric elements 34. Consequently the cylindrical elastic member 30 generates a torsional vibration in response to the application of a sinusoidal voltage to the piezoelectric elements 35 for the torsional vibration, and it generates a longitudinal vibration in response to the application of a sinusoidal voltage to the piezoelectric elements 34 for the longitudinal vibration.
On a driving face D, constituting an end face of the vibration element 31, a cylindrical movable element 26, constituting the relative movement member and rotatably supported, is maintained in contact under a suitable pressure.
FIG. 6 shows the development in time of an elliptical movement on the driving face D, in the vibration element 21 or 31 shown in FIGS. 4A, 4B, 5A and 5B, by the combination of the torsional vibration (T-mode) and the longitudinal (elongation-contraction) vibration (L-mode) generated in the vibration element 21 or 31.
As shown in FIG. 6, a given point on the driving face D generates an elliptical movement, by giving a phase difference of (1/4) .lambda. between the cycles of the torsional vibration T and those of the longitudinal vibration L, wherein .lambda. is the wavelength. Now a driving frequency f is assumed to correspond to an angular frequency .omega. (=2.pi.f). At a time t=(6/4).multidot.(.pi./.omega.), the displacement of the torsional vibration T is at a maximum at the left, while the displacement of the longitudinal vibration L is zero. In this state the movable member 26 is maintained in pressure contact, by means of an unrepresented pressurizing mechanism, with the driving face D of the vibration element 21 or 31.
Then, in a period from t=(7/4).multidot.(.pi./.omega.) through t=0 to t=(2/4).multidot.(.pi./.omega.), the torsional vibration T varies from the maximum at the left to the maximum at the right, while the longitudinal vibration L varies from zero to the maximum at the top and returns to zero. Consequently a given point on the driving face D of the vibration element 21 or 31 rotates to the right, while pushing the moving element 26, which is thus driven.
Then, in a period from t=(2/4).multidot.(.pi./.omega.) to t=(6/4).multidot.(.pi./.omega.), the torsional vibration T varies from the maximum at the right to the maximum at the left, while the longitudinal vibration L varies from zero to the maximum at the bottom side and returns to zero. Consequently the given point on the driving face D of the vibration element 21 or 31 rotates to the left while it is separated from the movable element 26, so that the movable element is not driven. Though being pressurized by the pressing member, the movable member 26 does not follow the contraction of the vibration element 21 or 31 because of a significant difference in natural frequency.
If the resonance frequencies of the torsional vibration and the longitudinal vibration are approximately same, the vibration element 21 or 31 simultaneously generate the torsional and longitudinal vibrations (degenerate state) to generate the elliptical movement on the driving face D, thereby generating a driving force.
In a state of generation of such elliptical movement, by approximately matching the frequency of the torsional vibration with the resonance frequency thereof and also approximately matching the frequency of the longitudinal vibration with the resonance frequency thereof, there is attained a resonance state whereby the elliptical movement on the driving face D is enhanced.
In such a vibration actuator of the different-mode degenerate type, the piezoelectric elements 24, 25 or 34, 35 are provided, along the axial direction of the cylindrical member 20 or 30, over the entire divided faces 22a, 23a or 32a, 33a. Stated differently, the piezoelectric elements 24, 25 or 34, 35 are present on the two nodal positions of the torsional vibration, across the loop position thereof.
In case of generating a torsional vibration and a longitudinal vibration by the piezoelectric elements mounted on the elastic member, the piezoelectric elements are in general provided in positions including the nodes of these vibrations, because such positioning enables effective generation of the torsional and longitudinal vibrations, as the node shows the maximum distortion displacement in each vibration.
However, in the vibration actuator of the different-mode degenerate type shown in FIGS. 4A, 4B, 5A and 5B, the piezoelectric elements are also provided in the loop positions of the torsional and longitudinal vibrations. Such loop positions only show a small distortion displacement. Consequently the piezoelectric elements provided in such loop positions do not effectively contribute to the generation of the vibration, and the energy supplied to such piezoelectric elements is not effectively utilized. For this reason the driving efficiency of the vibration actuator could not be improved.
The vibration actuator proposed in the aforementioned Japanese Patent Application No. 6-275022 generates an elliptical movement on the driving face D by combining a first-order longitudinal vibration and a first-order torsional vibration. Therefore, in order to position the piezoelectric elements on the mutually close nodal positions of such vibrations, the piezoelectric elements 24 or 34 for the longitudinal vibration and the piezoelectric elements 25 or 35 for the torsional vibration have to be mutually superposed and positioned between the divided faces 22a, 23a or 32a, 33a of the vibration element 21 or 31.
It is therefore necessary to assemble the vibration elements 21 or 31 by laminating and adhering respectively the two semicircular elastic halves 22, 23 or 32, 33, the piezoelectric elements 24 or 34 of two layers for the longitudinal vibration, the piezoelectric elements 25 or 35 of two layers for the torsional vibration, and three electrodes 27, 28, 29 or 37, 38, 39. Such increased number of components of the vibration element 21 or 31 complicates the assembling step at the adhesion, thus increasing the number of work steps required.
Also the vibration actuator proposed in the aforementioned Japanese Patent Application No. 6-275022 is insufficient in the reliability in the prolonged continuous drive, in the moisture resistance and in the temperature resistance, since the vibration element 21 or 31 is formed by adhesion of the components, involving a large number of points of adhesion.
Also in the vibration actuator proposed in the aforementioned Japanese Patent Application No. 6-275022, in generating the longitudinal and torsional vibrations in the vibration element 21 or 31, the torsional vibration is easier to generate than the longitudinal vibration. Therefore, the torsional vibration can be sufficiently generated even with the piezoelectric elements 25 or 35 for the torsional vibration being smaller respectively than those 24 or 34 for the longitudinal vibration. Smaller piezoelectric elements 25, 35 for the torsional vibration can correspondingly reduce the electrostatic capacitance, thereby decreasing the required input energy and thus improving the driving efficiency.
However, in the vibration actuator proposed in the Japanese Patent Application No. 6-275022, it is difficult to vary the size of the piezoelectric elements 25, 35 for the torsional vibration and of the piezoelectric elements 24, 34 for the longitudinal vibration, since they have to be mutually superposed.
Also, in case of maintaining the movable element 26 in pressure contact with the vibration element 21 (31) as shown in FIG. 3B, the movable element 26 has to be pressed in the direction of the longitudinal vibration. For this reason, the longitudinal vibration of the vibration element 21 (31) may be attenuated by such pressing of the movable element 26.
In order to minimize such attenuation of the longitudinal vibration resulting from the pressing of the movable element 26, it is required to increase the size of the piezoelectric elements 24 or 34 for the longitudinal vibration in comparison respectively with that of the piezoelectric elements 25 or 35 for the torsional vibration. However, in the vibration actuator proposed in the Japanese Patent Application No. 6-275022, it is difficult to vary the size of the piezoelectric elements 25 or 35 for the torsional vibration and of the piezoelectric elements 24 or 34 for the longitudinal vibration, because of the above-mentioned reason.